Cathode-ray tube in which screening electrodes are provided at the electron gun to produce a beam of uniform density over its cross section along its path to the display screen

ABSTRACT

A discharge system including a device such as a cathode-ray tube has an electron gun in which an accelerating electrode is placed close to a planar emissive surface of a cathode and is held at a small positive potential to provide an accelerating electric field having unipotential surfaces which are parallel to the electron emissive surface of the cathode. To screen the accelerating field having these unipotential surfaces from the further accelerating electrodes in the gun or in the cathode-ray tube a screening electrode or electrodes is placed in front of the accelerating electrode. In consequence the accelerating electric field close to the electron emissive surface is undistorted by the fields beyond the screening electrode(s), so that the electron beam is devoid of crossover and has a substantially uniform current density over its cross section.

lJnit States 'atent Douglas S. Hills C/t'll'llOlDlI-RAY TUBE IN WHICHSCREENING l-ll,l-lCTl'lOl)ES ARE PROVIDED AT THE liLll lC'llRON GUN TOPRODUCE A BEAM OF lJNll OltM DENSl'lY OVER ITS CROSS SECTION ALONG ITSPATH TO THE DISPLAY SCREEN 5 Claims, 4 Drawing Figs.

US. Cl 315/31, 313/82, 313/85, 315/15 Int. Cl H0lj 29/56 lFieldot'Search 315/31, 15,

References Cited Primary Examiner-Roy Lake Assistant Examiner-V.Lafranchi AttorneyGriffin, Branigan and Kindness ABSTRACT: A dischargesystem including a device such as a cathode-ray tube has an electron gunin which an accelerating electrode is placed close to a planar emissivesurface of a cathode and is held at a small positive potential toprovide an accelerating electric field having unipotential surfaceswhich are parallel to the electron emissive surface of the cathode. Toscreen the accelerating field having these unipotential sur faces fromthe further accelerating electrodes in the gun or in the cathode-raytube a screening electrode or electrodes is placed in front of theaccelerating electrode. In consequence the accelerating electric fieldclose to the electron emissive surface is undistorted by the fieldsbeyond the screening electrode(s), so that the electron beam is devoidof crossover and has a substantially uniform current density over itscross section.

CATHODE-RAY TUBE IN WHICH SCREENING ELECTRODES ARE PROVIDED AT THEELECTRON GUN TO PRODUCE A BEAM OF UNIFORM DENSITY OVER TS CROSS SECTIONALONG ITS PATH TO THE DISPLAY SCREEN This invention relates to electronguns for vacuum electric devices such as cathode-ray tubes and todevices including such electron guns.

A conventional cathode-ray tube comprises an evacuated envelope ofelectrically insulating material having at one end of internallyphosphor coated light-transmitting screen and at the other end anelectron gun capable of producing a beam of electrons which, whenincident upon the phosphor layer, is effective to excite it to anemission of generally visible optical radiation which is transmittedthrough the screen.

in most applications the excited area of the phosphor upon which theelectron beam is incident, is required to have the smallest possibledimension in at least one direction and the electron beam mustaccordingly be focused onto the screen phosphor to form, say, a spot, bysuitable electrostatic or electromagnetic means. The spot must be notonly of the smallest possible size for most applications, but shouldalso be sharply defined, that it is to say, the intensity of the spotmust be homogeneous and must very sharply fall to substantially zero atthe sport periphery.

Accordingly to one aspect of the present invention, an electron gunsuitable fore use in a vacuum electric device such as a cathode-raytube, comprises a cathode, accelerating means for producing adjacent theelectron emissive surface of the cathode an accelerating electric fieldhaving unipotential surfaces which are substantially parallel to thatsurface, together with electric screening means effective to preventdistortion of those unipotential surfaces as a result of electric fieldsproduced by further electrodes in the device.

According to a further aspect of the present invention, an electron gunsuitable for use in a vacuum electric device such as a cathode ray tube,comprises in sequence a cathode, a first apertured electrode being anaccelerating electrode uniformly spaced apart from the electron emissivesurface of the cathode and effective when positively biassed withrespect to the cathode, to produce an accelerating electric field havingunipotential surfaces which are substantially parallel to the cathodeemissive surface and to the first electrode, together with a secondelectrode being a control electrode spaced apart from and having anaperture aligned with that of the first electrode, the second electrodebeing arranged to electrically screen the space extending to the cathodeso as to prevent distortion of those unipotential surfaces.

By unipotential surface is meant a surface over which there are nopotential differences.

Suitably the electron emissive surface of the cathode and the adjacentunipotential surfaces are substantially planar.

The specified electric field configuration must be maintained at leastover the area of the apertures or of the utilized beam area as the casemay be to ensure operation of the gun according to the invention. Thismay be achieved by uniformly spacing the first electrode incloseproximity to the adjacent cathode surface area overlaps the area of theaperture to avoid electrostatic edge effects.

Electrons leaving the emissive surface of the cathode will tend normallyto cross the unipotential surfaces of an impressed accelerating electricfield. Accordingly, the present invention ensures that substantially allelectrons leaving the cathode surface are accelerated to tend to travelalong paths normally of this emissive surface to produce a beam in whichlateral velocity components are reduced to a minimum. Subsequentfocusing and scanning of a beam in which substantially the only velocitycomponents are those along the beam axis is thus considerably simplifiedand in particular requires a minimum energy input to produce therequired focusing convergence of the beam,

In addition, the electron beam launched into the tube will be derivedonly from substantially that area of the cathode surface underlying theaperture in the first and accelerating electrode. Considered togetherwith the velocity distribution of electrons in the beam produced by thefield, the beam launched into the tube by the electrode structure of theinvention will be of substantially homogeneous density and will alsohave an accurately and sharply defined periphery. This beam canaccordingly be focused to an image or sport having a luminous intensitywhich is both homogeneous and which falls sharply to substantially zeroat the spot periphery.

The beam having the characteristics hereinabove set forth willaccordingly also uniformly illuminate an aperture in a furtherelectrode, for example a final anode along the tube axis, enabling thisaperture to be sharply imaged by suitable focusing means and enabling avariably resolution to be obtained in a cathode-ray tube including thegun. The ability of such a tube to produce a sport or the like, which isan image of an aperture is a of considerable significance, since itenables both the ultimate size and shape of the spot to be controlledirrespectively of beam current and therefore electrode potentials in thetube including modulation depth. Such control is unobtainable in mostconventional guns, wherein the spot on the tube screen is an image of abeam crossover which varies considerably with beam current, modulationdepth and various other factors.

Accordingly the beam current from the electron gun of the invention canbe increased considerably in excess of those values usable inconventional electron guns and the aperture size and thereby the spotsize can be reduced to well below those values hitherto not possiblewithout excessive loss of spot intensity.

In, for example, a cathode-ray tube using a conventional electron gunthe maximum beam current for a given spot size say 0.5 mm. in diameteris normally no more than 250 microamperes whereas in the gun of thepresent invention this beam current can be increased to well above l.5ma. for the same spot size. This increased current density enables thespot size to be reduced to 0.1 mm. without reduction in the overall spotintensity.

in most applications the electric field configuration that is to say afield having substantially planar unipotential surfaces will bemaintained up to the second control electrode as a result of the abilityof this electrode to screen the space extending to the cathode, fromexternal electric fields generated by potentials applied to furtherelectrodes, i.e., anodes disposed further up the tube.

Accordingly, the gun of the invention permits continuous modulation ofthe beam without causing any significant degradation of its properties.Conveniently the ratio of diameter of the aperture in the secondelectrode to the thickness of the electrode is arranged to produce thisscreening effect. However, in the case where the diameter of theaperture is so large as to produce an undesirably electrode thickness,the aperture is conveniently covered with an electrically conducting andelectron permeable mesh effective to produce the screening effect.Alternatively a screening effect can be produced by two closelyspaced-apart thin electrodes, in which the spacing is not less than theaperture diameters. This combination is equivalent to a single thickelectrode.

Suitably the electron emissive surface and the accelerating electrodeare planar, and are spaced apart by a distance of about 0.005 inches,the potential on the accelerating electrode being commonly less than.+l0 volts with respect to the cathode. This accelerating electrode alsoeach be made effective when negatively biased with respect to thecathode to produce cutoff of the electrode beam through the aperturetherein. This facility is particularly useful when the electron gun isused in a cathode-ray tube intended for television application, as itenables flyback suppression to be easily obtained.

The control electrode which is effective to modulate the electron beampassing through the aligned apertures, conveniently is maintained at apotential within the range 0 to volts with respect to the cathode.

In the case where the beam control signal applied to the secondelectrode is of such magnitude as to produce thereon an appreciablepotential swing, which may produce undesirable distortion of theunipotential surfaces, the first electrode is also conveniently arrangedto produce a screening effect on the space extending to the cathode. Asuitable screening effect is produced by arranging the first electrodeto have a penetration factor of less than about 0.70, penetration factorp being defined as P=(E,/E where E is the potential difference betweenthe second control electrode and the cathode, and E is the potential onthe first electrode required to suppress the beam.

Suitably the electron gun includes a third electrode being anaccelerating electrode spaced apart from the control electrode andhaving an aperture aligned with those of the first and secondelectrodes. This third electrode is arranged to be more positivelybiased with respect to the cathode than the first or the secondelectrode and is effective to produce that cohesion in the electron beamwhich might otherwise be lost by scattering due to the relatively lowaccelerating potential on the first and/or the second electrode.Conveniently the third electrode is supplemented in this function by atleast a fourth apertured electrode spaced apart but electricallyconnected thereto.

Accordingly to a yet further aspect of the present invention, therefore,a cathode-ray tube comprises an evacuated envelope having at one end aninternally phosphor coated lighttransmitting screen and at the other endan electron gun capable of producing a beam of electrons onto thescreen, the electron gun comprising a cathode, a first aperturedaccelerating electrode uniformly spaced apart from the electron emissivesurface of the cathode and effective when positively biased with respectto the cathode, to produce an accelerating electric field havingunipotential surfaces which are substantially parallel to this electronemissive surface and to the first electrode, a second control electrodespaced apart from and having an aperture aligned with that of the firstelectrode, the second electrode being arranged to screen so as toprevent distortion of any unipotential surfaces between the secondelectrode and the cathode as a result of electric fields produced byfurther electrodes in the tube, together with beam-focusing meanseffective to produce an electric field intersecting the constrictionfree electron beam of substantially constant area which is produced bythe gun and having unipotential surfaces which are effective to focusthe beam onto any part of the screen area.

Preferably and in order to assist to control the focusing of the beam,the cathode-ray tube includes, means producing an electric field whichif effective to control the divergence or the convergence of theelectron beam. Such means are well known in the art.

Suitably the focusing means comprise any two or more substantiallycircumferentially continuous electrically conducting elements spacedapart along the axis of and surrounding the electron beam and preferablyare in the form of a graphite or other electrically conductive coatingon the inner surface of the tube envelope. Alternatively the focusingelectric field is produced by spaced-apart apertured electrodes adaptedto be energized at different potentials or by coaxial cylinders, bothwell known in the art.

In the case where a graphite coating is provided in the form of ahigh-resistance helix extending along the neck of the tube and effectivewhen suitably energized to accelerate the electron beam onto the screen,the electrically conducting elements conveniently comprise thoseadjacent end turns of the helix facing the cathode. If the helix is of aparticularly high resistance per turn, the resistance between theseadjacent first end turns may be adequate to produce therebetween apotential difference effective to provide unipotential surfaces at theend of the helix nearest the gun which are for example sufficientlyconvex to produce focusing of the beam. Alternatively the first turns ofthe helix may be electrically isolated from the adjacent turns in orderto produce a sufficient potential difference to produce the requiredconvexity of the unipotential surface.

In the case where a continuous graphite coating is provided on the innerof the surface of the tube envelope, the two conducting elements areconveniently produced by electrically isolating an annular end region ofthe coating facing the cathode.

The cathode-ray tube of the invention may, according to yet a furtheraspect, to be included in display apparatus the first electrode withrespect to the cathode and applying a more positive bias as well as amodulating potential onto the control electrode.

An embodiment of the invention will now be particularly described withrespect to the accompanying drawings in which:

FIG. ll. Is an exploded sectional side view of an electron gun accordingto the present invention; and

FIG. 2 ls a sectional side view of a cathode ray tube including theelectron gun shown in FIG. 1.

FIG. 3 Is a sectional view of the accelerating electrode 14.

FIG. 4 Shows a further embodiment of accelerating electrode 14.Referring now to the drawings and in particular to FIG. R thereof, thecathode-ray tube comprises an evacuated electrically insulating envelope2 having at one end a lighttransmitting screen 4 internally coated witha phosphor layer 6 capable of transmitting through the screen, radiationof a selective wavelength when excited by an incident electron beamproduced by a gun indicated generally at 8 and shown in exploded detailin FIG. l.

The electron gun 8 which is supported within the second end of theenvelope 2 by means well known in the art, includes a cathode 9 havingan electron emissive surface 10 which is electrically heated to emissionby an element 12.

Supported about 0.004 inches from the electron emissive surface H0 is anaccelerating electrode 114 comprising a solid metal plate about 0.005inches thick and provided with a central aperture 16 of about 0.020inches diameter. This accelerating electrode M is connected to asuitable terminal point outside the tube envelope and is positivelybiased 5 volts above the cathode potential. This electrode arrangementproduces in the space between the electrode 14 and the electron emissivesurface of the cathode 9, an electric filed which has substantiallyplanar unipotential surfaces parallel to the electrode 14 and thesurface 10. This electric field produces from the emissive surface 10,an electron current which traverses this space substantially normally ofthe emissive surface 10 over the whole of the area thereof and in whichthe current density also is substantially uniform. The utilized electronbeam current passing through the aperture 16 of the electrode Maccordingly is derived only from that corresponding area of the emissivesurface it) which underlies the aperture 116 and is therefore both ofsubstantially constant area and current density. Since the electrode 14during beam propagation is not used for modulation purposes and ismaintained at a constant accelerating potential, the current densityfrom the cathode 9 and particularly the current density of the beamemerging from the aperture l6 also remains not only substantiallyconstant in spite of subsequent modulation or deflection of thiselectron beam, but also homogeneous.

In order to modulate the electron beam emerging from the aperture 16, acontrol electrode 18 in the form of a metal plate provided with acentral aperture 119 is disposed about 0.005 inches from the electrode114. The thickness of this control electrode 18 in relation to thediameter of the aperture 19 is arranged to ensure electric screening ofthe space adjacent to the cathode 9 from any electric field produced byfurther electrodes in the tube and thereby to prevent distortion of theunipotential surfaces of the electric field in this space. In oneembodiment of the invention an aperture of 0.020 inches is provided in ametal plate 0.030 inches thick.

In the case where the modulation applied to electrode 18 is substantial,some distortion of the electric field in the space extending to thecathode 9 is likely to occur due to the corresponding electric fieldexcursions. This distortion is particularly undesirably in the spacebetween the electrode 14 and the cathode 9 and is to a large extentavoided by arranging the electrode M, for example by selecting thediameter to depth ration of the aperture therein, to have a penetrationfactor which is preferably less than 0.70. Alternatively the aperture inthe electrode M, if thin, can be covered with a suitable screening mesh.

Two further electrodes and 22 both 0.012 inches thick and provided withcentral aligned apertures 243 and 2b of 0.020 inches diameterrespectively are supported 0.040 inches and 0.140 inches from thecentral electrode M5. These electrodes which are each maintained at apositive potential of about 1,030 volts with respect to that of thecathode, are effective further to accelerate the beam emergent from thecontrol electrode 110 and to produce that cohesion of the beam whichmight otherwise be lost by scattering as a result of the lowaccelerating potential on electrode M.

In order to more exactly control the divergence of the beam emergingfrom the apertures 11d and 19, further spaced-apart electrodes 29 to 36each 0.012 inches thick and having central aligned apertures of 0.020inches diameter are sequentially spaced 0.040 inches from each otheralong the axis of the tube. The further electrodes 29, 30, 3E and 33 areelectrically connected together and to the further electrodes 20 and 22,whereby to be at the positive potential of 1030 v, whereas the furtherelectrodes 32, 3d, 35 and 3:6 only are connected together and aremaintained at a positive potential of about 320 v. This potentialdifference on the electrodes produce an electric field at the region ofthe aligned apertures which is effective to control the divergence ofthe electron beam passing through and emerging from the apertures inthese electrodes.

Electrode 1% is connected to a continuous graphite coating 30 within theneck 40 of the tube envelope which coating ter minates in and isconnected to, a graphite high resistance helix 42. The end of thishelix, which is nearest the gun is maintained at the potential of theelectrode as namely 320 volts, and is effective to produce furtheracceleration of the electron beam onto the screen 1 of the tube. Theother end of the helix is at a higher potential.

By this arrangement the beam can be brought to a force at the phosphor 6on the screen 4, without the use of any other internal or externalfocusing means.

Since the beam is of substantially constant cross section and undergoingno crossover or constriction, the focused beam area will be an imageonly of the aligned apertures in the electron gun, rather than of theconstricted area of the beam as in conventional hitherto used electronguns. Accordingly the focused beam area is limited only by that size andshape of the apertures which is compatible with the lowest permissiblebeam current for a required screen luminance. The apertures may becircular if a spot is required, or may be in the form of an elongateslot if a focused line is required. Since in this invention the focusedbeam area can be made accurately to reflect the aperture shape ratherthan the beam constriction of conventional electron guns, an accuratelyrectangular or square spot can be produced upon the tube screen. Thisrectangular spot if dimensioned in accordance with the picture pointsize in the line and in particular by the frame dimension of atelevision picture scan, can substantially reduce the visible effect ofthe scan on an image produced on the screen without significant loss ofdefinition.

At the screen end of the envelope 2, the helix is connected to a furthercontinuous conducting graphite coating Ml, for example of graphite oraluminum, which terminates at the screen ll.

The tube may be used in any suitable display apparatus including means,well known in the art, for applying the potentials hereinabove recited,necessary to operate the tube.

The cathode-ray tube of the invention can simply provide a focused beamarea of extremely small size and high current density which size remainssubstantially constant over wide variations in the beam current andposition on the tube screen and current density remains homogeneousthroughout the beam. In addition, due to the nature of the beam, thepower input necessary to scan the beam between different screenpositions and also to focus the beam is extremely low compared withconventional cathode-ray tubes. in for example the typical tubehereinabove described, a spot size of 0.010 inches diameter, thediameter of which remains substantially constant over the whole of thearea of the circular screen of 16 inches in diameter, can also bemaintained constant up to a beam current of 1 ma.

it will be appreciated that while the electron gun of the invention hasbeen described as used in a cathode-ray tube, it is also eminentlysuitable for use in any vacuum electric device such as a vidiconorthicon, travelling wave tube storage tube or even in devices merelyusing an electron beam in which a beam of the character described is ofadvantage.

It will be appreciated that while the invention has been described withrespect to a gun producing a single electron beam, it is equallyapplicable to guns producing a plurality of beams. A further advantageof the gun of the invention is that the spot size can be varied withoutdefocusing to produce a variable magnification. in use, a cathode-raytube incorporating the electron gun of the invention, will be operatedwith the electrode M permanently positively biased at a potential withrespect to the cathode and with the electrode 118 also permanentlypositively biased with respect to the electrode 14.

Either of these electrodes M and M can be connected to a terminal towhich modulation can be applied. Apparatus for or including such acathode-ray tube will accordingly include circuit means, well ltnown inthe art, to which the electrodes M and are convectible or connected andwhich are capable of maintaining these electrodes positively biased.

It will be noted that in order to ensure correct operation of the tubethe electrodes 20 and 22 must be capable of operation at voltages lowerthan that of the final anode voltage. These electrodes, 20 and 22 musttherefore be isolated within the tube from electrodes operating at ornear final anode voltage.

lclaim: ll. Electron beam producing system including a vacuum electronicdevice such as a cathode-ray tube, comprising:

an envelope; an electron gun mounted in said envelope, said guncomprising in succession a cathode having a planar electron emissivesurface, a first accelerating electrode immediately adjacent andparallel to the said emissive surface, said first accelerating electrodehaving therein a single aperture of smaller area than said electronemissive surface, at least one apertured screening electrode, and atleast one further electrode electrically separate from the screeningelectrode(s); means applying to the first accelerating electrode apositive potential relative to the cathode to produce adjacent theelectron emissive surface an accelerating electric field theunipotential surface of which are substantially parallel to that surfacein the space defined between said electron emissive surface and theaccelerating electrode; means applying potentials, higher than thatapplied to said first accelerating electrode, to the said at least onefurther electrode to provide a further accelerating electric fieldbeyond the said at least one screening electrode(s);

means applying a variable beam modulation potential to the screeningelectrode(s); the latter being effective to prevent penetration of saidspace by said further accelerating electric field, and the firstaccelerating electrode having a penetration factor as herein definedless than 0.7 to shield the said space from the effects of the variablebeam modulation potential applied to the screening electrode(s), wherebythe electron beam produced by the gun is devoid of crossover and ofsubstantially uniform current density over its cross section.

2. A system as claimed in claim ll wherein the apertured acceleratingelectrode is spaced from the adjacent cathode surface by a distance ofabout 0.005 inches.

the electron beam and includes a metallic mesh covering said aperture toproduce screening over the area of the aperture.

5. A system as claimed in claim 3, in which the ration of the diameterto the depth of the bore in the screening electrode is substantially2:3.

1. Electron beam producing system including a vacuum electronic devicesuch as a cathode-ray tube, comprising: an envelope; an electron gunmounted in said envelope, said gun comprising in succession a cathodehaving a planar electron emissive surface, a first acceleratingelectrode immediately adjacent and parallel to the said emissivesurface, said first accelerating electrode having therein a singleaperture of smaller area than said electron emissive surface, at leastone apertured screening electrode, and at least one further electrodeelectrically separate from the screening electrode(s); means applying tothe first accelerating electrode a positive potential relative to thecathode to produce adjacent the electron emissive surface anaccelerating electric field the unipotential surface of which aresubstantially parallel to that surface in the space defined between saidelectron emissive surface and the accelerating electrode; means applyingpotentials, higher than that applied to said first acceleratingelectrode, to the said at least one further electrode to provide afurther accelerating electric field beyond the said at least onescreening electrode(s); means applying a variable beam modulationpotential to the screening electrode(s); the latter being effective toprevent penetration of said space by said further accelerating electricfield, and the first accelerating electrode having a penetration factoras herein defined less than 0.7 to shield the said space from theeffects of the variable beam modulation potential applied to thescreening electrode(s), whereby the electron beam produced by the gun isdevoid of crossover and of substantially uniform current density overits cross section.
 2. A system as claimed in claim 1 wherein theapertured accelerating electrode is spaced from the adjacent cathodesurface by a distance of about 0.005 inches.
 3. A system as claimed inclaim 1 wherein the apertured screening electrode comprises an electrodehaving a bore to permit passage of the electron beam, the ratio of thediameter to the depth of the bore being effective to produce screening.4. A system as claimed in claim 1, wherein the apertured screeningelectrode has an aperture which permits passage of the electron beam andincludes a metallic mesh covering said aperture to produce screeningover the area of the aperture.
 5. A system as claimed in claim 3, inwhich the ratio of the diameter to the depth of the bore in thescreening electrode is substantially 2:3.